Microporous membranes comprising thermoplastic resins are used widely as a film for separation, selective permeation, or isolation of substances, and the like. For example, the usage includes battery separators for lithium ion rechargeable batteries, nickel-metal hydride batteries, nickel-cadmium batteries, or polymer batteries, separators for electric double-layer capacitors, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes and the like, moisture permeation waterproof clothes, medical materials and the like. In particular, a polyethylene porous membrane which exhibits ion permeability due to electrolytic solution impregnation, excellent electrical insulating properties, and a pore blocking effect, which blocks an electrical current to prevent an excessive temperature increase at a temperature of approximately 120 to 150° C. at the time of an abnormal temperature increase in a battery, is suitably used as a lithium ion rechargeable battery separator. However, if the temperature continues to increase even after the pore blocking for some reason, the membrane may be punctured due to the shrinkage of the membrane. This phenomenon is not limited to polyethylene porous membranes. Even in the case of a porous membrane using another thermoplastic resin, this phenomenon cannot be avoided at a temperature equal to or above the melting point of the resin constituting the porous membrane.
In particular, separators for lithium-ion batteries greatly affect battery characteristics, battery productivity and battery safety, and require good mechanical properties, heat resistance, permeability, dimensional stability, pore blocking characteristics (shutdown characteristics), membrane melt-puncture characteristics (melt-down characteristics) and the like. Furthermore, they require improved adhesion between a separator and an electrode material for an improvement in the cycle characteristics of batteries and improved electrolyte permeability for an improvement in productivity. For this purpose, research has been conducted to laminate various modified porous layers on porous membranes. Polyamideimide resins, polyimide resins, and polyamide resins, which have both good heat resistance and good electrolyte permeability, fluorine-based resins, which exhibit good electrode adhesion, and the like are preferably used as resins constituting modified porous layers. A modified porous layer described in the present invention refers to a layer that contains a resin which provides or improves at least one of the functions among oxidation resistance, adhesion to an electrode material, electrolyte permeability and the like.
Patent Document 1 discloses a composite porous membrane having a peel strength (T-peel strength) of from 1.0 to 5.3 N/25 mm at the interface between a polyethylene porous membrane and a coating layer. The membrane was produced by applying a varnish of polyvinylidene fluoride and inorganic particles (mass ratio: 15:85) to a polyethylene porous membrane having a thickness of 9 μm and allowing a portion of the polyvinylidene fluoride to moderately penetrate into the fine pores of the polyethylene porous membrane so as to exhibit an anchor effect.
Patent Document 2 discloses a separator having a heat-resistant porous layer containing a self-crosslinking acrylic resin and a plate-like boehmite provided on a corona discharge-treated polyethylene porous membrane with a thickness of 16 The resultant separator exhibited the peel strength (T-peel strength) of 1.1 to 3.0 N/10 mm at 180° between the polyethylene porous membrane and the heat-resistant porous layer.
In Working Example 1 of Patent Document 3, a polyethylene resin solution comprising: 50 parts by mass of a composition containing 47.5 parts by mass of polyethylene having a viscosity-average molecular weight (Mv) of 200,000, 2.5 parts by mass of polypropylene having an Mv of 400,000, and an antioxidant; and 50 parts by mass of liquid paraffin; is extruded from an extruder at 200° C. and withdrawn with a cooling roller kept at 25° C. to obtain a gel-like molded product, and the product is then stretched biaxially 7×6.4 times to obtain a polyolefin resin porous membrane. A multi-layer porous membrane obtained by laminating a coating layer that comprises polyvinylalcohol and alumina particles on the surface of the polyolefin resin porous membrane is disclosed.
In Working Example 6 of Patent Document 4, a polyethylene resin solution that comprises: 30 wt. % of a composition including polyethylene resins having a weight average molecular weight (Mw) of 4,150,000 and an Mw of 560,000, respectively, and a weight ratio of 1:9; and 70 wt. % of a mixed solvent of liquid paraffin and decalin; is extruded from an extruder at 148° C. and cooled in a water bath to obtain a gel-like molded product, and the product is then stretched biaxially 5.5×11.0 times to obtain a polyolefin resin porous membrane. A non-aqueous separator for a rechargeable battery obtained by laminating a coating layer that comprises meta-type all-aromatic polyamide and alumina particles on the surface of the polyethylene porous membrane is disclosed.
In Working Example 1 of Patent Document 5, 47 parts by mass of a homopolymeric polyethylene having a viscosity-average molecular weight (Mv) of 700,000, 46 parts by mass of a homopolymeric polyethylene having an Mv of 250,000, and 7 parts by mass of a homopolymeric polyethylene having an Mv of 400,000 were dry-blended using a tumbler blender. This example discloses a porous membrane obtained by: adding 1 wt. % of pentaerythrityl-tetrakis-[3(3,5-di-t-butyl-4-hydroxyphenyl)propionate] as an anti-oxidant to 99 wt. % of the obtained pure polymer mixture; melt-kneading the dry blended polyethylene composition once again using a tumbler blender; extruding and casting the composition onto a cooling roller controlled to a surface temperature of 25° C. so as to obtain a sheet-like polyolefin composition having a thickness of 2000 μm; biaxially stretching the polyolefin composition 7×7 times so as to obtain a polyethylene porous membrane; and applying an aqueous dispersion of baked kaolin and latex to the resultant polyethylene porous membrane.